Numerous technologies exist for measuring level of liquids or solids in an industrial process environment. Among these are transmitters which measure the level and transmit a signal representing actual level. The techniques for measuring level include guided wave radar, magnetostrictive, capacitance and the like.
A magnetic level indicator is another type of commonly used level sensing device. A magnetic level indicator, also known as a flipper gauge, is constructed of a chamber, a float and a visual indicator. The chamber, also known as a cage, is essentially a pipe or similar device external to a process tank or vessel which is usually mounted horizontally and which is usually connected to the tank through two or more horizontal pipes. One of the horizontal pipes is near the bottom of the chamber and the other is near the top of the cage. This arrangement allows the material level in the chamber to equalize with the material level in the tank, largely isolating the cage from agitation, mixing or other activities in the tank. The chamber, which is usually a pressure vessel, can be isolated from the tank using valves. The float is sized and weighted for the specific gravity and pressure of the application and contain magnets which actuate a visual indicator on the outside of the chamber to indicate level.
In certain applications it is desirable to transmit a level signal to a remote device in addition to the local visual indication of a magnetic level indicator. Currently, magnetic level indicators are used with magnetostrictive transmitters or with a series of reed switches, either of which provides an indication of continuous level which is redundant to the primary visual indication provided by the magnetic level indicator. Both the magnetostrictive and reed switch sensors are located on and external to the chamber and are actuated by the magnet placed inside the float in the chamber. A significant drawback to these redundant systems is that the float may fail, in which case both the primary visual and secondary transmitter signals are lost.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention, a redundant level measuring system includes a probe-type measurement instrument with the probe mounted in the chamber.
Broadly, there is disclosed herein a redundant level measuring system comprising a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A measurement instrument includes a probe and a measurement circuit. The instrument is mounted atop the chamber with the probe extending downwardly into the chamber. The measurement circuit measures a characteristic of the probe representing level of the material in the chamber. Shield means in the chamber isolate the float from the probe.
It is a feature of the invention that the probe comprises a coaxial probe having a signal rod contained in an outer tube and wherein the outer tube defines the shield means.
It is another feature of the invention that the shield means physically isolates the probe from the float.
Still another feature of the invention is that the shield means comprises an elongate screen extending longitudinally in the chamber providing an electromagnetic shield and the screen is disposed between the probe and the float. The screen comprises a ferrous metal screen. In one aspect of the invention the probe comprises a twin rod probe. In accordance with another aspect of the invention the probe comprises a single rod transmission line. In accordance with still a further aspect of the invention the probe comprises a transmission line and the screen comprises a return for the transmission line.
It is a further feature of the invention that the shield means comprises an electromagnetic shield cage housing the float in the chamber.
It is still a further feature of the invention wherein the probe comprises a capacitance probe.
There is disclosed in accordance with another aspect of the invention a redundant level measuring system comprising a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet-actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A guided wave radar measurement instrument includes a probe defining a transmission line. The instrument is mounted atop the chamber with the probe extending downwardly into the chamber. A measurement circuit is connected to the probe for generating pulses on the transmission line and receiving reflected pulses returned on the transmission line, the reflective pulses representing the level of the material in the chamber. Shield means in the chamber electromagnetically isolate the float from the transmission line.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.